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Regime-specific numerical solutions are valuable in studying the behaviour of reservoir processes both for practical field management and for benchmarking the success of analytical solutions within the regime of interest. In this work, a three-dimensional numerical reservoir simulator is developed for expansion-drive reservoirs. The governing equation is discretized using finite difference approach, method of conjugate gradient with the aid of MATLAB program is used to solve the resulting system of linear equations to obtain cell pressure at every time step. Cumulative production is computed as sum of expansion from each cell and oil production rate is determined until bubble point pressure is reached. The average reservoir pressure is determined as a weighted average based on the stock tank oil that is left in the reservoir, and finally the recovery factor at the bubble point pressure is computed. Contour plots, with colour map to ease the user's assimilation and interpretation of the simulator results, of reservoir pressure depletion with time were generated for different number of finite-difference grid blocks. The results indicate that the more the number of grid blocks used, the more accurate the numerical solution and the more detailed the description of the reservoir fluid distribution. The plot of average reservoir pressure against time shows a rapid decline in the average reservoir pressure due to the negligible compressibility associated with rock and liquid expansion-drive reservoirs. The estimated oil cumulative production of 236MSTB was recovered in 1180days up to the bubble point using the developed simulator. Furthermore, sensitivity analysis was performed to investigate the impact of key reservoir parameters on the average reservoir pressure. This work can be used to understand depletion in undersaturated reservoirs both for commercial field management and for validating new analytical solutions.